The diesters of naphthalenedicarboxylic acids are useful for preparing a variety of polymeric materials such as polyesters or polyamides. One particularly useful diester is dimethyl-2,6-naphthalenedicarboxylate (DM-2,6-NDC). Dimethyl-2,6-naphthalenedicarboxylate, for example, can be condensed with ethylene glycol to form poly(ethylene-2,6-naphthalate) (PEN), a high performance polyester material. Fibers and films made from PEN have considerably improved strength and superior thermal properties relative to, for example, poly(ethyleneterephthalate). For this reason, PEN is an exceptional material for preparing commercial articles such as thin films which can be used, for example, in the manufacture of magnetic recording tape and electronic components. Additionally, because of its superior resistance to gas diffusion, and particularly to the diffusion of carbon dioxide, oxygen and water vapor, films made from PEN are useful for manufacturing food containers, especially the so-called "hot fill" food containers. PEN can also be used to prepare high strength fibers useful for the manufacture of, for example, tire cord.
In order to prepare high quality PEN suitable for commercial use, it is necessary to start with purified DM-2,6-NDC. The purified DM-2,6-NDC must be low in color, substantially free of organic and inorganic impurities, and low in particulate matter.
DM-2,6-NDC is most readily prepared by the esterification of 2,6-naphthalenedicarboxylic acid (2,6-NDA) with methanol. The 2,6-NDA is conveniently prepared by the liquid phase, heavy metal catalyzed oxidation of a 2,6-dialkyl- or 2-alkyl-6-acyl naphthalene compound using molecular oxygen as the source of oxygen for the oxidation reaction. During this oxidation reaction impurities such as 6-formyl-2-naphthoic acid (FNA), trimellitic acid (TMLA) and, when a bromine oxidation promoter is used, various brominated compounds are produced. Although in some instances it would be desirable to use 2,6-NDA directly for the preparation of PEN, however, because of its high melting point (&gt;300.degree. C. with decomposition) and extremely low solubility in ordinary solvents, 2,6-NDA is extremely difficult to purify to acceptable levels by standard purification techniques such as distillation and recrystallization. These difficulties in purifying 2,6-NDA are partially overcome by converting 2,6-NDA to its dimethyl ester, DM-2,6-NDC. DM-2,6-NDC can be distilled and it can be recrystallized from solvents such as methanol or from one or more aromatic solvents. However, even though DM-2,6-NDC can be purified by treatments such as distillation or recrystallization, purifying DM-2,6-NDC to a purity acceptable for use in the aforementioned manufactured articles remains a problem in the art. For example, the FNA produced during the oxidation of dialkylnaphthalene is incorporated (as a methyl ester) into DM-2,6-NDC during the esterification of 2,6-NDA and is very difficult to remove or reduce to acceptable low levels. Oxidation catalyst metals such as cobalt and manganese when used with a bromine promotor for the preparation of 2,6-NDA are also typically carried over into the esterification reaction as impurities. This is because a certain amount of the oxidation catalyst metal is complexed tightly to TMLA and other oxidation by-products and is not removed in the oxidation mother liquor when the oxidation mother liquor is separated from the solid 2,6-NDA. Catalyst metals cause problems in the downstream operations used for purifying the DM-2,6-NDC by, for example, causing a thickening of the distillation bottoms and plugging of the distillation column.
Additionally, during the esterification reaction, the mono methyl ester of 2,6-naphthalene dicarboxylic acid, hereinafter referred to as mono-methyl-2,6-naphthalenedicarboxylate (MM-2,6-NDC), is produced and it must be removed from the DM-2,6-NDC. Furthermore, it is highly desirable to remove most of the MM-2,6-NDC from the DM-2,6-NDC before it is distilled. Otherwise, because it is a high melting solid, it causes an excessively large amount of distillation bottoms, and it can precipitate in and foul the distillation apparatus. The ratio of methanol to 2,6-NDA in the esterification reaction determines the amount of MM-2,6-NDC produced. When the ratio is high, for example greater than 10:1 by weight, respectively, only a small amount of MM-2,6-NDC is formed. However, when a low ratio of methanol to 2,6-NDA is used, for example 4:1, as much as 6 to 10 weight percent of the esterification product is MM-2,6-NDC. Using high ratios of methanol to 2,6-NDA in order to avoid the formation of MM-2,6-NDC is not, however, desirable for a large, commercial-scale operation. High ratios of methanol to 2,6-NDA necessitate using a large esterification reactor for a given throughput of product. Additionally, by using a high ratio of methanol to 2,6-NDA, large quantities of methanol must be recycled and the loss of DM-2,6-NDC is greater due to the amount of DM-2,6-NDC remaining in solution in the extra methanol.
The present invention allows for the use of low ratios of methanol to 2,6-NDA in the esterification by providing for the effective separation of MM-2,6-NDC from DM-2,6-NDC. Furthermore, in the process of this invention, the separated MM-2,6-NDC is relatively free of undesirable impurities and it is therefore recyclable to the esterification reactor where it is converted to DM-2,6-NDC, resulting in no loss of valuable product.
Finally, especially for applications where the DM-2,6-NDC will be used for making PEN for very thin films, particulate contamination in the DM-2,6-NDC must be eliminated or reduced to very low levels. These particulate impurities, which range in size down to below 1.5 microns, can arise from a variety of sources. For example, they may be either oxidation catalyst particles or, if used, esterification catalyst particles. They may also be derived from filtering and drying operations where DM-2,6-NDC is dissolved in a solvent, recrystallized, separated from the recrystallization mother liquor by filtration and dried to remove excess solvent. Inevitably, particulates contaminate the DM-2,6-NDC product in these processes. Regardless of the source, particulate contamination in the DM-2,6-NDC product is undesirable.
Processes for manufacturing and purifying DM-2,6-NDC have been disclosed. Japanese Kokai Patent No. Sho 50-116461, for example, discloses a process for preparing DM-2,6-NDC wherein crude DM-2,6-NDC from the esterification of 2,6-NDA with methanol is distilled and then crystallized from methanol. This process is taught as being superior to one where the crystallization from methanol precedes the distillation. However, the Japanese Kokai Patent No. Sho 50-116461, although disclosing a process for preparing DM-2,6-NDC from 2,6-NDA by reaction with methanol, does not disclose the advantages of using low ratios of methanol to 2,6-NDA. It also does not teach a suitable means for removing large amounts of MM-2,6-NDC from DM-2,6-NDC. Additionally, this Kokai patent teaches that it is essential to recrystallize the DM-2,6-NDC subsequent to a distillation step in order to prepare DM-2,6-NDC with acceptable color. However, this order of the purification steps does not provide for low levels of particulates in the final DM-2,6-NDC product that is required for some of the aforementioned uses of DM-2,6-NDC.
The art needs a process for the large-scale preparation of DM-2,6-NDC having suitably low color and low levels of impurities such as the methyl ester of FNA, bromine containing impurities and, especially, MM-2,6-NDC and particulate contaminants. The art also needs a process for efficiently removing MM-2,6-NDC from DM-2,6-NDC so that reduced ratios of methanol to 2,6-NDA can be used in the esterification reaction. The present invention provides such a process.